Drilling for fossil fuels in deepwater targets can be expensive and risky if not well planned and prepared. Proper well planning requires reliable anticipation of geohazards. One such geohazard in deepwater drilling is shallow water flow (“SWF”) sands, which are highly porous sands that are prone to flowing when drilled. If a deep-sea drill permeates these SWF layers, the sands can flow and cause extensive damage to the borehole and the well site. SWF layers have cost the oil industry hundreds of millions of dollars to date. Detection of the SWF layers, therefore, is important for reducing both commercial loses and environmental risks.
FIG. 1 is a representation of the current understanding of the formation of a SWF. SWF sands can occur in water depths between 300 m and 600 m below the mud line 101. They are found all over the world in areas where loose and unconsolidated sediments with high sedimentation rates create overburden layers 102. A low permeability seal 103 and a layer of shale or mudstone 105 underlie the SWF sands 104. This underlying zone 103 and 105 is the condensed section where sediments are compacted and the rate of sedimentation is low. If there are isolated sand bodies 104 within this shale or mudstone 105, then the water from such bodies will not escape easily due to the presence of low permeability sediments 105 around them. Additionally, the high rate of sedimentation from the overburden 102 can exert an enormous pressure on these sediments, causing these isolated bodies with large amounts of water to be over-pressured 106.
When a drill bit punctures the SWF layer, the resulting over-pressured sand 104 can flow at the wellhead and pose drilling, environmental, and health problems. For example, SWF can cause drilling template-blowouts, buckling of casing, loss of well or wellhead, costly downtime in rig, and can result in leakage of hydrocarbon in the seabed, which can cause severe environmental damage.
Reliable detection of potentially hazardous SWF situations is key to controlling the problem and for managing the associated costs.
It has been common practice to use pore pressure to locate potential SWF sands. Pore pressure can be predicted before drilling from conventional seismic stacking velocities with a normal compaction trend analysis using, for example, the well-known Eaton approach. Velocities that appear to be slower than “normal velocities” are indicative of overpressure, which then is quantified using an empirical equation.
There are several problems with the conventional approach. First, conventional seismic stacking velocities are usually unsuited for pressure prediction because they are not rock or propagation velocities. Second, these velocities lack resolution in depth. Third, in a deepwater environment, sediment loading often has been so fast that fluid pressures in these sediments are above hydrostatic below the mud line. This prevents development of a normal compaction trend; thus invalidating the entire approach in deepwater.